This invention relates generally to gas turbine engines and more particularly to a turbofan engine in which the fan flow can be modulated independent of the fan speed.
Future mixed mission morphing aircraft as well as more conventional mixed mission capable military systems that have a high value of take-off thrust/take-off gross weight, i.e., a thrust loading in the 0.8-1.2 category, present many challenges to the propulsion system. They need efficient propulsion operation at diverse flight speeds, altitudes, and particularly at low power settings where conventional engines operate at inefficient off-design conditions both in terms of uninstalled performance and, to an even greater degree, fully installed performance that includes the impact of spillage drag losses associated with supersonic inlets.
When defining a conventional engine cycle and configuration for a mixed mission application, compromises have to be made in the selection of fan pressure ratio, bypass ratio, and overall pressure ratio to allow a reasonably sized engine to operate effectively at both subsonic and supersonic flight conditions. In particular, the fan pressure ratio and related bypass ratio selection needed to obtain a reasonably sized engine capable of developing the thrusts needed for combat maneuvers and supersonic operation are non-optimum for efficient low power subsonic flight. Basic uninstalled subsonic engine performance is compromised and fully installed performance suffers even more due to the inlet/engine flow mismatch that occurs at reduced power settings.